The HIV-1 envelope contains multiply overlapping (and redundant) mechanisms of humoral immune defence. These including glycan shielding, conformational masking, and immunodominant decoys with variable loops. Although explored extensively, many of these mechanisms are still only partially understood. We are using structure-based methods to define them further. In particular, we have investigated the conformation of unliganded gp120, differences between SIV and HIV-1, early evasion of low-titer neutralizing antibodies, and evasion by HIV-2.